Range lists, zero-length functions, linker gc

[Fangrui Song]

On 2020-05-31, Fangrui Song wrote:
>It is being discussed on llvm-dev
>(https://lists.llvm.org/pipermail/llvm-dev/2020-May/141885.html https://groups.google.com/forum/#!topic/llvm-dev/i0DFx6YSqDA)
>what linkers should do regarding relocations referencing dropped functions (due
>to section group rules, --gc-sections, /DISCARD/, etc) in .debug_*
>
>As an example:
>
>  __attribute__((section(".text.x"))) void f1() { }
>  __attribute__((section(".text.x"))) void f2() { }
>  int main() { }
>
>Some .debug_* sections are relocated by R_X86_64_64 referencing undefined symbols (the STT_SECTION
>symbols are collected):
>
>  0x00000043:   DW_TAG_subprogram [2]
>                  ###### relocated by .text.x + 10
>                  DW_AT_low_pc [DW_FORM_addr]     (0x0000000000000010 ".text.x")
>                  DW_AT_high_pc [DW_FORM_data4]   (0x00000006)
>                  DW_AT_frame_base [DW_FORM_exprloc]      (DW_OP_reg6 RBP)
>                  DW_AT_linkage_name [DW_FORM_strp]       ( .debug_str[0x0000002c] = "_Z2f2v")
>                  DW_AT_name [DW_FORM_strp]       ( .debug_str[0x00000033] = "f2")
>
>
>With ld --gc-sections:
>
>* DW_AT_low_pc [DW_FORM_addr] in .debug_info are resolved to 0 + addend
>  This can cause overlapping address ranges with normal text sections. {{overlap}}
>* [beginning address offset, ending address offset) in .debug_ranges are resolved to 1 (ignoring addend).
>  See bfd/reloc.c (behavior introduced in
>  https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=e4067dbb2a3368dbf908b39c5435c84d51abc9f3 )
>
>  [0, 0) cannot be used because it terminates the list entry.
>  [-1, -1) cannot be used because -1 represents a base address selection entry which will affect
>    subsequent address offset pairs.
>* .debug_loc address offset pairs have similar problem to .debug_ranges
>* In DWARF v5, the abnormal values can be in a separate section .debug_addr
>
>---
>
>To save your time, I have a summary of the discussions. I am eager to know what you think
>of the ideas from binutils/gdb/elfutils's perspective.
>
>* {{reserved_address}} Paul Robinson wants to propose that DWARF v6 reserves a special address.
>  All (undef + addend) in .debug_* are resolved to -1.
>
>  We have to ignore the addend. With __attribute__((section(".text.x"))),
>  the address offset pair may be something like [.text.x + 16, .text.x + 24)
>  I have to resolve the whole (.text.x + 16) to the special value.
>
>  (undef + addend) in pre-DWARF v5 .debug_loc and .debug_ranges are resolved to -2
>  (0 and -1 cannot be used due to the reasons above).
>
>* Refined formula for a relocated value in a non-SHF_ALLOC section:
>
>   if is_defined(sym)
>      return addr(sym) + addend
>   if relocated_section is .debug_ranges or .debug_loc
>      return -2   # addend is intentionally ignored
>
>   // Every DWARF v5 section falls here
>   return -1  {{zero}}
>
>* {{zero}} Can we resolve (undef + addend) to 0?
>
>  https://lists.llvm.org/pipermail/llvm-dev/2020-May/141967.html
>
>  > while it might not be an issue for ELF, DWARF would want a standard that's fairly resilient to
>  > quirky/interesting use cases (admittedly - such platforms could equally want to make their
>  > executable code way up in the address space near max or max - 1, etc?).
>
>  Question: is address 0 meaningful for code in some binary formats?
>
>* {{overlap}} The current situation (GNU ld, gold, LLD): (undef + addend) in .debug_* are resolved to addend.
>  For an address offset pair like [.text + 0, .text + 0x10010), if the ending address offset is large
>  enough, it may overlap with a normal text address range (for example [0x10000, *))
>
>  This can cause problems in debuggers. How does gdb solve the problem?
>
>* {{nonalloc}} Linkers resolve (undef + addend) in non-SHF_ALLOC sections to
>  `addend`. For non-debug sections (open-ended), do we have needs resolving such
>  values to `base` or `base+addend` where base is customizable?
>  (https://lists.llvm.org/pipermail/llvm-dev/2020-May/141956.html )

Forgot to mention

* {{compatibility}} Do we need an option if we change the computed value of (undef + addend) to
   -2 (.debug_loc,.debug_ranges)/-1 (other .debug_*)
   (or 0 (other .debug_*), but it might not be nice to some binary formats {{reserved_address}})

   https://lists.llvm.org/pipermail/llvm-dev/2020-May/141958.html

   > If we end up blessing it as part of the DWARF spec, we probably
   > wouldn't want it to be user-configurable for the .debug_ sections, so
   > I'd hesitate to add that configurability to the linker lest we have to
   > revoke it to conform to DWARF (breaking flag compatibility with
   > previous versions of the linker, etc). Admittedly we'll be breaking
   > output compatibility with this change regardless, so potentially
   > having the flag as an escape hatch could be useful.

   I hope we don't need to have a linker option. But if some not-so-old
   versions of gdb / binutils programs / elfutils programs can't cope
   with  -2/-1/0 {{reserved_address}}, we may have to invent a linker option.

   I hope GNU ld, gold and LLD can have a compatible option.
   (As an LLD contributor, I'd be happy to implement the opinion in LLD)